Genotypic evaluation of twenty-eight high- and low-cyanide cassava in low-land tropics, southeast Nigeria

Emmanuel Ukaobasi Mbah,Blessing Chinwoke Nwankwo,Damian Ndubuisi Njoku,Michael A Gore

doi:10.1016/j.heliyon.2019.e01855

Emmanuel Ukaobasi Mbah, Blessing Chinwoke Nwankwo + Show 2 more

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https://doi.org/10.1016/j.heliyon.2019.e01855

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Abstract

A two-year field experiment was carried out in a randomized complete block design with two replications in 2015/16 and 2016/17 cropping seasons at the National Root Crops Research Institute, Umudike (05° 29′N; 07° 33′E; 122 m above sea level) in Nigeria. The objectives of the study were to assess growth, disease status and yield responses of twenty-eight (28) newly developed high- and low-cyanide cassava genotypes in low-land humid tropics of Umudike, Nigeria. Plant height, stem girth, canopy diameter, number of leaves/plant, cassava mosaic disease (CMD) and cassava bacterial blight (CBB) incidence and severity as well as bulking rate and fresh root yield varied significantly (P < 0.05) amongst the high- and low-cyanide cassava genotypes in both cropping seasons. Also, the results showed that bitter cassava genotypes exhibited greater tolerance to CMD than sweet cassava. However, there was no significant (P > 0.05) difference in bulking rate and fresh root yield between the two groups. The Pearson's and Spearman's ranked associations between fresh root yield of the cassava genotypes and other variables analysed across the two cropping seasons were highly significant (P ≤ 0.01) and positive contrary to the other variables. However, they exhibited different degrees of associations amongst themselves, especially CMD incidence that indicated highly significant and positive association with severity. The principal component analysis across the two cropping seasons indicated eigen-values of the four axes > unity with cumulative variance of 68.98 %. Most of the characters that contributed to the 22.35 % observed variability in principal component (PC1) were CMD incidence and severity, and number of leaves/plant while PC2 also exhibited high vector load from plant attributes such as number of leaves/plant, bulking rate ha−1 and canopy diameter. The bi-plot clustering indicated that genotypes (BI-56, NR110439 and B1-29) exhibited strong similarity amongst themselves across the tested variables. The combined fresh root yield sequence of the first ten high yielder genotypes was in the order: NR110439 > TMS010354 > NR110315 > NR 110238 > NR 110228 > NR 060169 > BI-117 > BI-50 > NR110084 > NR 110181. These cassava genotypes were considered to be better endowed genetically, hence their improvement can be encouraged to ensure high and sustainable root yield. A poly-linear and positive regression was recorded between CMD and root yield as well as between CBB and root yield indicating that they affected fresh root yield of high- and low-cyanide cassava genotypes and demands attention also in cassava improvement studies.

Highlights

Cassava (Manihot esculenta Crantz), which is a crop that is very tolerant to drought and heat stress produces well on marginal soils (Alves, 2002; Calle et al, 2005; Dixon et al, 2008) and serves as a staple food crop in South-eastern Nigeria
Cultivars of cassava are generally classified as bitter or sweet cyanide depending on the level of the two cynogenic glucosides (CG) present in the plant parts (Siritunga and Sayre, 2003); such that on enzymatic hydrolysis they release cyanohydrin and free-hydrocyanic acid (HCN) (Cardoso et al, 2005; Njoku and Ano, 2018)
The results further showed that highly significant (P 0.01) and positive relationships were obtained between stem girth and canopy diameter as well as cassava mosaic disease (CMD) incidence and CMD severity with correlation coefficients (r) of 0.54 and 0.86, respectively

Summary

Introduction

Cassava (Manihot esculenta Crantz), which is a crop that is very tolerant to drought and heat stress produces well on marginal soils (Alves, 2002; Calle et al, 2005; Dixon et al, 2008) and serves as a staple food crop in South-eastern Nigeria. Cultivars of cassava are generally classified as bitter (high-) or sweet (low-) cyanide depending on the level of the two cynogenic glucosides (CG) (linamarin, which accounts for 80 % of CG and lotaustralin) present in the plant parts (Siritunga and Sayre, 2003); such that on enzymatic hydrolysis they release cyanohydrin and free-hydrocyanic acid (HCN) (Cardoso et al, 2005; Njoku and Ano, 2018). In contrast to sweet (low-cyanide) cassava roots which are processed by peeling and boiling or roasting, bitter (high-cyanide) cassava roots demand a more extensive processing method that goes in sequential order: peeling, washing, grating, fermenting, drying or frying, among others to reduce the HCN content to a safe level for human consumption. According to World Health Organization (WHO), the safe level for cyanide in cassava flour is 10 ppm or 10 mg HCN kgÀ1 (FAO/WHO, 1991; Cardoso et al, 2005)

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